Parameterization of contrail radiative properties for climate studies

The study of contrails and their impact on global climate change requires a cloud model that statistically represents contrail radiative properties. In this study, the microphysical properties of global contrails are statistically analyzed using collocated Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) observations. The MODIS contrail pixels are detected using an automated contrail detection algorithm and a manual technique using the brightness temperature differences between the MODIS 11 and 12 mm channels. The scattering and absorption properties of typical contrail ice crystals are used to determine an appropriate contrail model to minimize the uncertainties arising from the assumptions in a particular cloud model. The depolarization ratio is simulated with a variety of ice crystal habit fractions and matched to the collocated MODIS and CALIOP observations. The contrail habit fractions are determined and used to compute the bulkscattering properties of contrails. A parameterization of shortwave and longwave contrail optical properties is developed for the spectral bands of the Rapid Radiative Transfer Model (RRTM). The contrail forcing at the top of the atmosphere is investigated using the RRTM and compared with spherical and hexagonal ice cloud models. Contrail forcing is overestimated when spherical ice crystals are used to represent contrails, but if a hexagonal ice cloud model is used, the forcing is underestimated for small particles and overestimated for large particles in comparison to the contrail model developed in this study.